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Re: [Xen-devel] [PATCH for-4.5 v7 05/21] xen: Relocate p2m_access_t into common and swap the order

On Thu, Sep 18, 2014 at 11:22 AM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
>>> On 17.09.14 at 22:51, <tklengyel@xxxxxxxxxxxxx> wrote:
> We swap the order of the enum of types n ... rwx, as to have rwx at 0. This
> allows
> us to memset the permission store to 0, without having to specify rwx as the
> default
> permission when memaccess is not in use. This also has performance benefits
> for
> non-memaccess paths, as now comparison is to 0 when checking if memaccess is
> in use,
> which is usually faster.

I'd say often instead of usually here.


And then the description leaves open whether any dependency on
rwx now being first is getting introduced (no memset() is being added
here, yet wherever an explicit one would get added a respective
BUILD_BUG_ON() should be put along with it).


I guess the commit message is a bit ambiguous, the paths I see all set mem_access explicitly afterwards to default_access, so there is no reliance on pure memset initializing it right now. This patch really just opens the door to have such an approach in the future (if needed).


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